Premixed Salt and Internal Standard Solution Streamlines Forensic Blood Alcohol Testing

Blood alcohol concentration is among the most frequently performed measurements in all of forensic toxicology, underpinning decisions in impaired-driving prosecutions, accidental-death investigations, and a wide range of medico-legal proceedings. Because a single number can help determine whether a person goes to prison, the analytical methods used to produce it must be not only accurate but also rugged enough to survive the pressures of routine casework. A new study from the Department of Forensic Toxicology at South Korea’s National Forensic Service, published in the open-access journal Results in Chemistry, tackles a deceptively simple question: can the sample preparation steps behind this cornerstone test be safely reduced without compromising the numbers that end up in court?

The analytical workhorse in question is headspace gas chromatography coupled with flame ionization detection, universally abbreviated HS-GC/FID. The technique exploits a fundamental physical property of ethanol: it is highly volatile and partitions favorably between a liquid sample and the gas phase above it. In practice, a small volume of blood is sealed in a vial, heated to a controlled temperature so volatile analytes escape into the headspace, and a measured portion of that gas is injected onto the chromatographic columns. Flame ionization detection then quantifies the compounds with high sensitivity and excellent reproducibility. Dual-column configurations, in which each sample is run through two columns of different selectivity, add a layer of qualitative confirmation by requiring that ethanol produce consistent results on both phases.

Many forensic laboratories enhance this process with a phenomenon known as salting-out. Adding a high concentration of an inorganic salt such as sodium chloride, ammonium sulfate, or potassium carbonate reduces the solubility of volatile compounds in the aqueous matrix, driving more ethanol molecules into the headspace and sharpening the analytical signal. In the conventional workflow examined by the researchers, designated Method I, a technician pipettes saturated sodium chloride solution into a 10-milliliter headspace vial, then adds an internal standard solution of tert-butanol, and finally adds the whole blood sample before analysis. The internal standard is critical: because tert-butanol behaves similarly to ethanol during headspace partitioning and chromatography but is absent from human blood, the ratio of the ethanol signal to the tert-butanol signal cancels out much of the variability in injection volume and instrument response.

That three-step pipetting sequence, however, is a hidden vulnerability. When laboratories process large batches of blood alcohol samples under time pressure, every additional manual pipetting step is an opportunity for operator-dependent variability, transcription slips, and lost seconds that accumulate across hundreds of cases. The Korean team, led by Miyeon Lee and Young-Hoon Jo, proposed a streamlined alternative, Method II, in which the saturated sodium chloride solution and the tert-butanol internal standard solution are premixed into a single reagent that can be aliquoted into vials in advance and stored under refrigeration. At analysis time, the technician simply adds the blood sample to the premixed solution, vortexes briefly, and lets the vial equilibrate at room temperature before the autosampler takes over. The tert-butanol concentration in the final mixture is unchanged; only the workflow is different.

To find out whether this simplification carried an analytical price, the researchers compared the two methods across a blood alcohol concentration range of 0.33 to 3.33 grams per liter, spanning the low values relevant to administrative thresholds and the high values seen in serious intoxication cases. Calibration relied on aqueous ethanol reference standards from Cerilliant at concentrations of 10, 50, 100, 200, 300, and 400 milligrams per deciliter. Crucially, the conventional Method I was performed independently by two different analysts, allowing the team to separate genuine method differences from ordinary operator variability. The study used real forensic casework blood samples from living subjects, handled under approval from the National Forensic Service’s Institutional Review Board.

The results were striking in their agreement. Across 21 paired measurements from the first analyst, the mean relative difference between Method I and Method II was 101.5 percent, with individual values ranging from 96.0 to 106.4 percent of the comparison value. Across an extended dataset of 39 samples from the second analyst, agreement was even tighter, averaging essentially 100 percent. In that larger dataset, inter-method differences remained consistently below the study’s acceptance threshold, and 81 percent of the first analyst’s samples showed differences below 0.010 grams per liter. Bland-Altman analysis, a statistical technique that plots the difference between paired measurements against their mean and computes 95 percent limits of agreement, confirmed that the two methods tracked each other closely across the full concentration range, with no systematic drift over sampling periods extending from 7 to 133 days.

Stability of the premixed reagent was the other make-or-break question, since a solution that degrades in the refrigerator would be worse than useless. The team stored premixed sodium chloride and internal standard solutions at 4 degrees Celsius and tested aliquots at 0, 1, 2, 3, and 4 months. The measured blood alcohol values showed no meaningful change over that period, indicating that the tert-butanol internal standard remains chemically compatible with saturated brine during extended cold storage. The authors note that minor variations observed after prolonged storage at room temperature were small and did not affect analytical validity, underscoring that proper refrigeration, rather than any inherent flaw in the chemistry, is the key handling requirement.

The most legally consequential validation came from certified reference materials: commercially prepared human whole-blood samples with traceable ethanol concentrations established by multiple laboratories accredited to ISO 17025. The researchers analyzed two such materials, certified at approximately 1.0 and 3.0 grams per liter, in triplicate using both methods at 3, 7, 17, and 60 days. At every time point, Method II results fell within the established forensic confidence intervals and closely matched both the certified values and the conventional method’s results. Notably, comparable performance persisted even at 60 days, meaning a laboratory could prepare a bulk batch of premixed vials and rely on it for two months without re-preparation, a substantial efficiency gain for high-throughput operations.

What makes this study compelling is its modesty of ambition. The researchers did not attempt to improve the fundamental sensitivity or selectivity of ethanol determination, which headspace GC/FID already delivers at a level few techniques can match. Instead, they targeted the procedural layer where human hands meet analytical chemistry, showing that reducing three pipetting steps to two measurably shrinks the surface area for error while leaving the underlying science untouched. In forensic contexts, where even small variations in manual preparation can propagate into analytical variability and, ultimately, into contested evidence, standardized and simplified workflows are a quiet but essential form of quality assurance.

The implications extend beyond South Korea. Salting-out-based headspace protocols remain embedded in standard operating procedures at forensic laboratories worldwide, and many of them still rely on sequential addition of salt and internal standard solutions. For those laboratories, the message of this study is that a premixed internal standard and sodium chloride reagent, stored refrigerated and used within a validated window, offers a practical path to faster batch processing, fewer operator-dependent errors, and smoother integration with autosamplers and future automation. It is a reminder that in forensic science, robustness is not only about the instrument on the bench but about every pipette stroke that precedes it, and that sometimes the most valuable innovation is simply doing one fewer thing correctly.

Subject of Research: Simplified premixed internal standard and sodium chloride sample preparation for forensic blood alcohol analysis by headspace gas chromatography

Article Title: Improving workflow robustness in forensic blood alcohol analysis using a premixed internal standard–NaCl solution for HS-GC/FID

Article References: Improving workflow robustness in forensic blood alcohol analysis using a premixed internal standard–NaCl solution for HS-GC/FID. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: forensic toxicology, blood alcohol concentration, headspace gas chromatography, flame ionization detection, salting-out, internal standard, tert-butanol, sodium chloride, method validation, certified reference materials, workflow robustness, National Forensic Service